Camming differential with oil pump means

ABSTRACT

A differential mechanism (10) having two output cam members (16, 17) rotatable about an axis (A) each cam member having an annular cam surface of undulating form comprising pairs of mutually inclined surfaces (24, 25; 26, 27). The inclined surfaces are engaged by end surfaces (29, 30) of cam followers (28) so that relative contra rotation of the output cam members (16, 17) causes the cam followers to slide axially. An input element (11-14) engages and supports the followers (28) and moves the followers circumferentially relative to the output cam members and a component (11, 14) of the differential which rotates during use of the differential drives oil pump means (70) for forcing oil into the differential.

The invention relates to a differential mechanism particularly but notexclusively for use in motor vehicles.

Differential mechanisms commonly used on vehicles are of the sun andplanet gear type and have a well known disadvantage that when one wheelis on a slippery surface such as mud or ice and the other wheel is on afirm surface capable of providing traction, the first wheel will simplyspin as it receives all the available power transmitted to thedifferential.

Limited slid differential mechanisms have been proposed in an attempt toovercome this problem which restrict the extent to which one wheel canspin relative to the other but such differentials are more complex and,therefore, more costly to produce.

In European patent application EP-A-0284329 there is proposed analternative differential mechanism comprising two output cam membersrotatable about an axis, with each cam member having a single annularcam surface thereon of undulating form comprising pairs of mutuallyinclined surfaces, and a plurality of cam followers having end surfacesengaging the cam surfaces of the output cam members, the arrangementbeing such that relative contra rotation of said output cam memberscauses the cam followers to slide axially, and an input element engagingand supporting the followers and moving the followers circumferentiallyrelative to the output cam members.

Such a differential will hereinafter be called a differential of thetype described.

It is desirable to lubricate the relatively moving surfaces within sucha differential to keep wear to a minimum.

According to the invention there is provided a differential of the typedescribed, characterised in that a component of the differential whichrotates during use of the differental drives oil pump means for forcingoil into the differential.

The oil pump means may comprise scoops arranged externally on thecomponent which rotates and connected with oil massageways through thecomponent.

The component which rotates and which drives the oil pump means may bethe input element of the differential.

In an alternative construction the oil pump means comprises an oil pump.

The oil pump may conveniently comprise a non-rotatable housingcontaining an impellor mounted on an outer surface of the input member.

Also according to the invention there is provided a differential of thekind described, wherein each output cam member has oil passagewaysformed therein.

The invention also provides a method of manufacture of a steel cammember having a cam surface thereon, wherein the whole member is casehardened and is subsequently treated by a salt bath nitrocarburizingprocess, and thereafter selected surfaces of said cam member areinduction hardened.

Differential mechanisms in accordance with the invention will now bedescribed by way of example with reference to the accompanying drawingsin which:

FIG. 1 is a cross section through a differential mechanism in accordancewith the invention taken through output cam members;

FIG. 2 is an end view of the differential of FIG. 1 shown partly brokenaway;

FIG. 3 is a development of cam surfaces with cam followers shown inpositions therebetween;

FIG. 4 is a diagrammatic end view of a cam follower;

FIG. 5 is an enlarged section through a single output cam member and theadjacent portion of the input element as shown in FIG. 1 taken on theline V--V of FIG. 7;

FIG. 6 is an isometric view of an output cam member as shown in FIG. 5;

FIG. 7 is an end face view of the input element shown in FIG. 5 showingthe oil pump means;

FIG. 8 is a section through a portion or an alternative input elementwith integral oil pump means;

FIG. 9 is an isometric view of the portion of the input element shown inFIG. 8;

FIG. 10 is a section through a modified form of differential;

FIG. 11 is a view on the line XI--XI of FIG. 10 showing lubricationgrooving details;

FIG. 12 is a section through a portion of an alternative input elementwhich drives an oil pump, and

FIG. 13 is a view on line XIII--XIII of FIG. 12 showing details of theoil pump housing and impellor.

In FIGS. 1 to 4 the differential 10 is housed in a surrounding casing(not shown) partially filled with oil and comprises a drive inputhousing 11 in the form of a gear 12 which receives drive from a pinion(not shown) in known manner. The gear 12 is drivably connected to hubs13 and 14 which are held in the housing by any suitable means such asscrewing into the housing 11 and then locking in position by anysuitable means such as peening, welding or circumferentially spacedbolts.

Two output cam members 16, 17 each have a central bore 62 with splines15 therein, to drive output shafts (not shown) extending through bores18 in the hubs 13, 14. The bores 18 each have a helical oil feed groove19 on the inner surface thereof which, when the differential is in use,feed lubricant from the casing into the differential.

The output cam members 16, 17 are rotatable in bearings 50 about an axisA relative to the hubs. The output members 16, 17 have respectiveflanges 20, 21 thereon on which are formed respective undulating facecams 22, 23. The cam 22 comprises an annular zigzag surface (shown indetail in FIG. 3) made up from seven pairs of mutually inclined helicalsurfaces 24, 25. The cam 23 also comprises an annular zigzag surface,apparent from FIG. 3, but is made up from eight pairs of mutuallyinclined helical surfaces 26,27. As shown in FIG. 1, the undulating camsurface 22 is inclined at an angle P to the axis A and the cam surface23 is inclined at angle P2 to the axis A whereby each cam surfaceconverges towards the other.

Fifteen cam followers 28 are positioned between the cams 22,23. Each camfollower is of strut-like elongate form and comprises two sets ofmutually inclined end surfaces 29,30 and 32, 33 which terminate atrelatively longer side surfaces 34,35. The angle of inclination Qbetween the end surfaces 29,30 corresponds to the angle of inclinationbetween the cam surfaces 24,25 and the angle of inclination S betweenthe end surfaces 32,33 corresponds to the angle of the inclinationbetween the cam surfaces 26,27. The end surfaces 29,30 are also inclinedat angle P and the end surfaces 32,33 are inclined at angle P2 asapparent from FIG. 1. The angles P and P2 may be the same.

When viewed from the end each cam follower is arcuate which enables thefollowers to be assembled together as viewed in FIG. 2. Each camfollower has an arcuate embrace of substantially 360/nf degrees where nfis the number of cam followers. However, if desired, the arcuate embracemay be less to leave clearance spaces 28' (See FIG. 4) between thefollowers.

Each cam follower includes an elongate drive dog 36 having mutuallyinclined side surfaces 37,38 (FIG. 4). The drive dogs 36 locate withslight clearance 36a in complementary shaped grooves 39 formed in theinner periphery of a cylindrical drive input element 40 formed on inputhousing 11. The clearance 36a is just sufficient to ensure that thearcuate outer periphery (indicated at 28a) of each follower 28 can abutthe inner peripheral surface (40a) of the drive input element 40. Thegrooves 39 provide support for the followers 28 at least adjacent theiraxial ends and preferably, as shown, for substantially their entirelength.

As apparent from FIG. 2 and 3, the assembly of the cam followers ispreferably such as to place the side surfaces 34, 35 of adjacentfollowers so that they interengage or lie closely adjacent. In that waymaximum use is made of the available circumferential space for the camfollowers, the followers together forming a substantially continuous andcompact annular array as viewed in FIG. 2.

When driven input is applied through drive input element 40, andassuming that a vehicle having the differential is being driven in astraight line, the cam followers apply a load to the surfaces of cams22, 23 so as to rotate the output members 16, 17 at equal speeds. Asapparent from FIG. 3, alternate followers are in driving engagement withsurfaces 24,26 of the cams. However intermediate cam followers havetheir surfaces in non-driving engagement with surfaces 25,27 of thecams.

The driving force applied by the followers 28 to the inclined surfaces24,26 produces a reaction force F as illustrated in FIG. 4. Theinclination of the end surfaces of the cam followers at angle P and P2causes the application of force F to create an outward force G. FIG. 4shows the forces for cam 22 with an end surface inclination of angle P.Forces F and G produce a resultant force R which passes radiallyoutboard of edge E preferably approximately through or adjacent a cornerC1 between the drive dog 36 and an adjacent outer peripheral part 40a ofthe follower 28. In that way the loading on the cam follower tends towedge it firmly against a corner C2 of the drive input element 40 insuch a way that tipping of the follower about its edge E is avoided.

The differential effect can best be appreciated considering the drivingelement 40 as being stationary and by applying contra rotary movement tothe cams 22,23 in directions J,K respectively in FIG. 3. The camsurfaces 26 will move to the left and cam surfaces 24 to the right. Suchmovement of the cam surface 26 causes the associated follower to moveaxially towards cam 22. If both cams 22,23 and the drive input element40 are all given an additional rotational movement in direction of arrowJ, it will be appreciated that the cams 22 and 23 will be rotatingrespectively faster and slower than element 40. The difference in speedsbetween the two cams 22,23 and the drive input element 40 will resultfrom the different number of cam surfaces on the cams. As there is aconsiderable amount of friction between the followers and the cams,torque will be transmitted to one cam even when the other is drivablyconnected to a wheel spinning on a slippery surface, which is highlyadvantageous over conventional differential systems.

The moving of one wheel faster than the other will result in a reductionin net torque applied to that wheel through the associated cam due tothe load applied by the axially moving cam followers, to which inputtorque is applied. There will be, in that case, an increase in the hentorque applied at the other cam and the ratio between the hen torqueswill be dependent upon the values of the angles Q,S. The greater theangles, the greater will be the friction at the cam surfaces due noaxial loading applied thereto by the followers. The angles Q,S arenormally selected whereby the face cams may drive the cam followers, butthe cam followers cannot drive the face cams. However, if desired, theangles Q,S can be selected to provide a degree of reversibility.

As mentioned above, the adjacent cam followers may be arranged withtheir side surfaces 34,35 closely adjacent or in inter-engagement,driving force F applied to any follower 28 in non-driving engagementwith cam surfaces may be arranged to transmit driving load appliedthereto to the next driving follower through the interengaging surfaces.Also the use of interengaging surfaces further inhibits the camfollowers tipping relative to the cams.

Interengagement of the surfaces will take place over substantially theirentire length.

The torque ratio requirement between the two net torques of the insideand outside output shafts is higher in some applications than others.Where high torque differentials are required say from 3:1 to 5:1 thiscan cause problems with wear rates between the cam followers and the camsurfaces.

Axial thrust applied to the cams by the followers is transmitted to thehub members 13 and 14 through the back face 52 of the cam members 16,17via the thrust washers 51 and needle bearings 53. Shims may be used toadjust the relative axial positions of the cams. A belleville washer 54(shown compressed into a flat configuration in FIG. 1) may be arrangedto act against a bearing washer 55 to urge the followers 28 into firmengagement with the cams 22,23. The urging of the followers against thecams also creates a radially outward force Z on the followers 28resulting from the angles of inclination P and P2 and in addition helpsto reduce backlash.

The needle bearings 53 could be replaced by thrust washers.

Now with reference also to FIGS. 5,6 and 7, both cam members 16,17, onlyone of which is shown for convenience, have axially extending oilpassageways 61 arranged on the internal surfaces of the central bore 62.The passageways 61 pass the whole length of the bore 62 and at one endthereof open radially into the splines 15.

A second set of oil passageways 64 connect the face cam surface 23 tothe backface 52 of the cam member 17. The passageways 64 open into thetroughs between pairs of mutually inclined surfaces 24,25.

The face cam surface 23 can be surface treated by peening the surface byshot blasting before any subsequent hardening treatment. The shotblasting lasts for upto 1 minute with spring steel cut wire shot, of alength of 0.7 mm and a diameter of 0.7 mm. This has the effect ofcreating little oil reservoirs on the cam surface.

After case hardening the surface may be further treated by anitrocarburizing process. The surfaces of the output cam members 16 and17 are treated by a salt bath nitrocaburizing process. A suitableprocess is the "SURSULF" process (trade mark of Hydromechanique etFrottement, France). Alternatively the cam members 16 and 17 may betreated by nitro carburizing only after case hardening.

After nitrocarburizing the back faces 52 of the cam members 16,17 areinduction hardened up to a hardness of Rockwell 60.

At least one of the hubs 13 and 14, is provided on its axially outerexternal surface 71 with an oil pump means 70. The oil pump means isformed from a sheet metal conical pressing 75 with at least one shelllike oil scoop 72, and preferably six scoops, which pump oil into thedifferential housing 11. The scoops 72 are connected to the interior ofthe housing 11 by oil passageways 73 which pass axially through the hubmember 14 and provide oil flow for passageways 64 and radially throughneedle bearings 53 to locations radially outboard of flanges 20 and 21.Both hub members 13 and 14 may be provided with similar oil pump means70.

In another embodiment of the invention shown in FIGS. 8 and 9, thescoops 72A are integrally formed with the input member 14. In this casethe scoops 72A are basically radial abutments which push the liquidtowards the passageways 73.

FIGS. 10 and 11 show a modified construction which the rear faces 52 ofthe output cam member 16 and 17 are provided with an annular groove 16a,17a which links up the passageways 64. Chordally disposed grooves 16b,17b are also provided on the rear face of each output member to link upthe annular grooves 16a, 17a the passageways 64 and annular oil volumes78, 79 radially outboard of the cams. As will be appreciated grooves16a, 17a; 16b,17b promote the flow of oil whenever output members 16 and17 rotate relative to the surrounding components.

FIG. 12 and 13 show a further alternative embodiment of the presentinvention in which the external scoops 72,72A of the previousembodiments are replaced by a pump 80 which surrounds and is driven fromthe hub 14.

Pump 80 comprises a non rotatable housing having an inner part 81 and anouter part 82 and an impellor having a central cylindrical band 83 whichencompasses the hub 14 and vanes 84 which extend radially outwardly fromthe band at circumferentially spaced locations. The impellor is mouldedfrom rubber or plastics material so that vanes 84 are flexible.

As can be seen from FIG. 13 a pick-up pipe 85 which receives oil fromwithin the surrounding casing provides the inlet to pump 80 and also, byits connection with a non-rotating portion of the differential, resistsany tendency for the pump housing to rotate.

An exhaust port 86 from the pump housing is connected with an annularcollection reservoir 87 from which extend passageways 88 which are theequivalent of passageway 73 in the earlier embodiments. Between inletpipe 85 and exhaust port 86 the cross section of the pump decreases at87 to produce the change in volume which provides the pumping action.The pump vanes flex, as shown at 84a, when passing through the decreasedcross-section portion 87 of the pump.

The cylindrical band portion 83 of the impellor may be a friction gripon limb 14 or may be positively driven by interengaging formations onthe band and hub or by other fasteners between the band and hub.

The pump 80 is designed to fill the differential with oil and maintain asteady flow of oil through the differential on rotation of housing 11.Oil flowing through passageways 88 enters groove 17a and passageways17b,64 in the adjacent output cam member 17 (see FIGS. 10 and 11) toflow to the cam surfaces 22,23 via passageways 64 and reach the oilvolumes 78,79 radially outboard of the cams via bearings 53. Oil thenflows radially inwardly through passageways 64 in the other output cammember 16 to again reach cam surfaces 22,23. Oil exits from thedifferential radially inwardly between cams 22,23 where it flows axiallyoutwardly along passageways 61 and helical feed grooves 19.

We claim:
 1. A differential mechanism (10) comprising two output cammembers (16, 17) rotatable about an axis (A), each said member having anannular cam surface thereon of undulating form comprising pairs ofmutually inclined surfaces (24, 25, 26, 27), and a plurality of camfollowers (28) having end surfaces (29, 30) engaging the cam surfaces ofthe output cam members, the arrangement being such that relative contrarotation of said cam members (16, 17) causes the cam followers to slideaxially, and an input element (11-14) engaging and supporting thefollowers (28) and moving the followers circumferentially relative tothe output cam members, characterised in that the output cam members(16, 17) have oil passageways (61, 64) formed therein.
 2. A differentialmechanism (10) comprising two output cam members (16,17) rotatable aboutan axis (A), each said member having an annular cam surface thereon ofundulating form comprising pairs of mutually inclined surfaces(24,25,26,27), and a plurality of cam followers (28) having end surfaces(29,30) engaging the cam surfaces of the output cam members, thearrangement being such that relative contra rotation of said output cammembers (16,17) causes the cam followers to slide axially, and an inputelement (11-14) engaging and supporting the followers (28) and movingthe followers circumferentially relative to the output cam members,characterized in that a component (11,14) of the differential whichrotates during use of the differential drives oil pump means (70) forforcing oil into the differential and wherein the oil pump means (70)comprises scoops (72) arranged externally on the component (11,14) whichrotates and is connected to oil passageways (73) through the component(14).
 3. A differential as claimed in claim 2 wherein the componentwhich rotates and which drives the oil pump means (70) is the inputelement (11,14) of the differential.
 4. A differential as claimed inclaim 3 wherein the scoops (72A) are formed integrally with the inputelement (14).
 5. A differential as claimed in claim 3 wherein the outputcam members (16,17) are connected to output drive shafts which passthrough bores (18) in the input element (13,14), and said bores eachhave a helical oil feed groove (19) therein.
 6. A differential asclaimed in claim 2 wherein the output cam members (16,17) have oilpassageways (61) formed therein.
 7. A differential as claimed in claim 6wherein the two output cam members (16,17) each have splines (15) at thecentre thereof for driving output drive shafts, and there is at leastone axially extending oil passageway (61) also arranged at the centre ofeach cam member and which passes through the splines.
 8. A differentialas claimed in claim 6 wherein the annular cam surface on each output cammember (16,17) is connected to the oil pump means (70, 72) by axial oilpassageways (64) extending from at least some of the troughs betweenpairs of inclined cam surfaces to an axially outer surface (52) of therespective output cam member.
 9. A differential as claimed in claim 8wherein the axially outer surface (52) of each output cam member (16,17)is provided with an annular groove (16a, 17a) which provides an oil flowlink between the axially outer ends of each axial oil passageway (64).10. A differential as claimed in claim 9 wherein the axially outersurfaces (52) of each output cam member (16,17) is provided withchordally disposed grooves (16b,17b) which provide an oil flow linkbetween the annular groove (16a,17a) and the outer periphery of eachoutput cam member (16,17).
 11. A differential as claimed in claim 2wherein the oil pump means comprises an oil pump (80) driven by thecomponent (14) which rotates.
 12. A differential as claimed in claim 11in which the oil pump (80) delivers oil to at least some of the troughsbetween pairs of inclined cam surfaces (24,25;26,27) via axiallyextended passageways (64) in the output members.
 13. A differential asclaimed in claim 11 wherein the oil pump (80) surrounds and is drivenfrom part of the input member (14).
 14. A differential as claimed inclaim 13 wherein the oil pump (80) comprises a non-rotatable housing(81,82) containing an impellor (83,84)) mounted on an outer surface ofthe input member.
 15. A differential as claimed in claim 14 wherein thehousing (81,82) has an inlet (85) and an outlet (86) and the crosssection of housing decreases (87) between the inlet and outlet toproduce the pumping action.
 16. A differential as claimed in claim 15wherein the pump outlet (86) is connected with an annular collectionreservoir (87) which encircles the input member (14), the reservoirbeing in turn connected with axially extending passageways (88) in theinput member.
 17. A differential as claimed in claim 14 wherein theimpellor comprises a central annular band (83) which encompasses theouter surface of the input member (14) and a plurality of air vanes (84)extending radially outwardly from the band at circumferentially spacedlocations thereon.
 18. A differential as claimed in claim 17 wherein thepump vanes (84) are formed from rubber or plastics material and bend asthey pass through the decreased cross section (87) of the housing(81,82).
 19. A differential as claimed in claim 14 wherein the pumpinlet is provided by an inlet pipe (85) which is secured at one end to anon-rotatable component and at the other end to the pump housing(81,82), said inlet pipe thus also serving to prevent rotation of thehousing during use of the differential.
 20. A differential mechanism(10) comprising two output cam members (16,17) rotatable about an axis(A), each said member having an annular cam surface thereon ofundulating form comprising pairs of mutually inclined surfaces(24,25,26,27), and a plurality of cam followers (28) having end surfaces(29,30) engaging the CAM surfaces of the output cam members, thearrangement being such that relative contra rotation of said output cammembers (16,17) causes the cam followers to slide axially, and an inputelement (11-14) engaging and supporting the followers (28) and movingthe followers circumferentially relative to the output cam members,characterized in that a component (11,14) of the differential whichrotates during use of the differential drives oil pump means (70) forforcing oil into the differential and wherein the oil pump meanscomprises an oil pump (80) driven by the component (14) which rotates.21. A differential as claimed in claim 20 wherein the output cam member(16,17) have oil passageways (61) formed therein.
 22. A differential asclaimed in claim 21 wherein the two output cam members (16,17) each havesplines (15) at the center thereof for driving output drive shafts, andthere is at least one axially extending Oil passageway (61) alsoarranged at the center of each cam member and which passes through thesplines.
 23. A differential as claimed in claim 21 wherein the annularcam surface on each output cam member (16,17) is connected to the oilpump means (70,72) by axial oil passageways (64) extending from at leastsome of the troughs between pairs of inclined cam surfaces to an axiallyouter surface (52) of the respective output cam member.
 24. Adifferential as claimed in claim 23 wherein the axially outer surface(52) of each output cam member (16,17) is provided with an annulargroove (16a,17a) which provides an oil flow link between the axiallyouter ends of each axial oil passageway (64).
 25. A differential asclaimed in claim 24 wherein the axially outer surfaces (52) of eachoutput cam member (16,17) is provided with chordally disposed grooves(16b,17b) which provide an oil flow link between the annular groove(16a,17a) and the outer periphery of each output cam member (16,17). 26.A differential as claimed in claim 20 wherein the output cam members(16,17) are connected to output drive shafts which pass through bores(18) in the input element (13,14), and said bores each have a helicaloil feed groove (19) therein.